1. Field of the Invention
This invention relates to a steel bar or rod having a non-circular cross section for use in concrete reinforcement.
2. Description of the Prior Art
Steel bars having a non-circular cross section have been widely used in lieu of round bars as reinforcing bars for reinforcing concrete in the field of civil engineering and construction. While some of these steel bars have only either transverse ribs of varying shapes disposed around the outer circumference thereof so as to extend in the circumferential direction, or longitudinal ribs of varying shapes extending in the longitudinal direction, others have both transverse and longitudinal ribs as disclosed, for example, in U.S. Pat. Nos. 2,374,827 and 2,377,980.
In conjunction with the progress made in devising techniques for saving labor in the construction and development of light-weight construction materials, prefabricated reinforcing bar assemblies of a non-circular cross section have been produced on a massive scale and have gained wide application in recent years. The mass-production of the prefabricated reinforcing steel bar assemblies has succeeded in the drastic reduction in the time required for fabricating a concrete construction, and the improvement in the working efficiency, as well as the standardization of the reinforcing steel bars.
The only one principal problem left unsolved with the reinforcing steel bars of this type is the coupling or joint as a connecting structure between two similar reinforcing steel bars. The ideal coupler would be one that can connect similar reinforcing steel bars easily and rapidly, compensate for a considerable assembly error as well as a dimensional error, and yet ensure an accurate and reliable connection. These requirements are of utmost importance especially for the reinforcing steel bars having a non-circular cross section. Though there have to this date been proposed various coupling structures such as a thread-coupling system, a pressure-sleeve system and the like, all of them are directed to be adapted to commercially available reinforcing steel bars of a non-circular cross section and hence, are not perfectly satisfactory.
To cope with the above mentioned problem, a specific coupling structure has recently come to be provided for a reinforcing steel bar having a non-circular cross section per se so as to thereby simplify the coupler means, and tools, as well as the operations involved for accomplishing the coupling. As a typical example of the reinforcing bars of this type, mention can be made of a so-called threaded reinforcing steel bar wherein a number of lug-forming independent transverse ribs are disposed around the outer circumference of the bar in such a manner that groups of these transverse ribs can mate with a female helical thread.
As shown in FIGS. 1 and 2 more specifically, a number of transverse ribs 2 are disposed on the outer circumference of the bar 1 at every pre-selected pitch in the longitudinal direction thereof. Each of these transverse ribs 2 has a lug-forming shape extending from the core of the bar 1 and is assorted into two groups of ribs 2a, 2b which are disposed on the diametrically opposite surface portions of the core of the bar 1. The groups of the ribs 2a, 2b are disposed in the arrangement such that lugs of the ribs can mate with female threads sequentially. The reinforcing steel bars of this construction shown in FIGS. 1 and 2 are disclosed respectively in U.S. Pat. Nos. 3,782,839 and 3,561,185.
Because of the mating engagement with female threads, this type of reinforcing steel bar can be connected easily with each other by the use of a coupler-type sleeve and have an excellent threaded fixation and exhibit a reliable coupling action. In addition, tenacity of the reinforcing steel bar itself is satisfactorily high.
As shown in the enlarged sectional view of FIG. 3, the cross-sectional shape of the core of the conventional threaded reinforcing steel bar 1 is substantially round except for the groups of the transverse ribs 2. As shown in this Figures, the two groups of the transverse ribs 2 are disposed at opposed positions on the outer circumferential plane of the core of the steel bar. In this FIG. 3, the upper half of the group of ribs is shown to have a top portion which is concentric with the core while the lower half thereof is shown possessed of an oval bottom portion which has a roundness unlike the roundness of the core.
In this instance, the transverse ribs having the top portion are disposed so as to define an independent lug-form extending in the circumferential direction of the steel bar. Because of the coupling by means of the mating engagement with the female threads, however, no longitudinal rib is formed on the outer circumferential plane of the core so as to extend linearly in the longitudinal direction of the steel bar 1. In FIG. 3, the symbol 0 stands for the longitudinal axis of the core of the steel bar 1.
The reinforcing bar of the above-described type is generally produced by a hot-rolling process in order to satisfy the requirement of standardized mass-production. According to this process, a steel bar material is first hot pressed so as to possess an oval or a flat hexagonal cross section and is then passed through a pair of finishing rolls having a caliber as illustrated in FIGS. 4 and 5, whereby the steel bar is roll-shaped so as to have a cross section as shown in FIG. 3. The portion 3 of the caliber indicated by the full line in FIG. 4 is provided so as to shape the cubic outer circumferential surface of the steel bar into a circular form while the portion 4 thereof indicated by the dotted line is provided so as to shape the transverse ribs 2 having a top portion which is concentric with the core shown in FIG. 3.
Likewise the portion 4a indicated by the dotted line in FIG. 5 is provided so as to shape the transverse ribs 2 having an oval bottom portion shown in FIG. 3.
FIG. 6 illustrates the relation between the steel bar material 1a and a pair of finishing rolls 5, 5. During the roll-shaping operation of the steel bar material using the pair of finishing rolls 5, 5 having the caliber shown in FIGS. 4 and 5, the material 1a having an oval or a flat hexagonal cross section is compressed from the top and bottom whereby a swelling section is formed in the transverse direction. Using this swelling section, the cross section of the core of the steel bar 1 is shaped into a roundness simultaneously with the formation of the transverse ribs 2. The reference numeral 6 represents a gap between the pair of rolls 5, 5.
For this reason, it is necessary to strictly control the cross-sectional area of the steel bar, the rolling shape thereof, the gap between the rolls, and the rolling speed. Even if all these conditions are perfectly satisfied, the effective length of the transverse rib 2 (in the circumferential direction of the steel bar 1) remains only in the range of about 1/3 to 1/2 of the circumferential length of the steel bar 1 in view of the essential conditions for the reinforcing steel bars of this type such as the bonding power of the concrete and the fixation of the threads in the coupling sections.
In other words, the effective length of the transverse rib 2 is considerably shorter when contrasted with that of the heretofore known reinforcing steel bars of a non-circular cross section in general. Namely, the threaded bar of the above-described type yet leaves unsolved problems with respect to the bonding power of the concrete and the stable and reliable fixation of the screw engagement.